1. Field of the Invention
The present invention relates to a hydraulic damper for use in a front fork for a motorcycle, and more particularly to a vertical hydraulic damper having an oil lock mechanism.
2. Description of the Relevant Art
Various hydraulic dampers for use in front forks for motorcycles are known. For example, U.S. Pat. No. 4,561,669 discloses a vertical hydraulic damper including an outer tube having an upper end closed by a fork cap and coupled to a motorcycle frame by means of a pair of upper and lower brackets. An inner tube is slidably inserted, from below, into the outer tube and has a lower end closed by a clamp member by which a wheel axle is rotatably supported. A cylinder or damper piston tube disposed concentrically within the inner tube has a lower end fixed to the axle clamp member for movement with the inner tube. A piston rod having an upper end attached to the fork cap extends in the cylinder and has a lower end on which there is mounted a piston held in sliding contact with the inner peripheral surface of the cylinder.
The disclosed hydraulic damper also includes an oil lock piston disposed on the upper end of the piston damper rod and projecting downwardly, and an oil lock cylinder mounted on a guide bushing fixed to the upper end of the cylinder and slidably fitted over the piston rod. When the hydraulic damper is compressed a maximum stroke, the oil lock piston is fitted into the oil lock cylinder to entrap working oil in the oil lock cylinder for locking oil to prevent bottoming of the hydraulic damper.
Since the oil lock mechanism is positioned on the upper end of the vertical hydraulic damper, the oil lock cylinder at the upper end of the cylinder must be filled with working oil at all times in order for the oil lock mechanism to operate reliably without fail. Since, however, a gas is filled in an upper portion of the damper, the oil lock cylinder may not be filled with a sufficient amount of working oil, causing the oil lock mechanism to malfunction or initiating oil locking operation at irregular positions. The oil lock cylinder has an oil lock chamber which has a small pressure-bearing area because a oil spring is disposed around the cylinder. Inasmuch as oil is not locked until a high pressure buildup is developed in the oil lock chamber, therefore, it is necessary to minimize the gap or clearance between the oil lock piston and the oil lock cylinder. At times, the oil lock piston and the oil lock cylinder may be brought into contact with each other, thus producing metal particles due to wear. The metal particles thus produced tend to damage the sealing capability between the cylinder and the piston for generating damping forces. As a result, the generated damping forces are liable to become unstable.